1. Field of the Invention
The invention relates to a method of producing sterically stabilized aqueous dispersions of polymers using macromonomers which are esterification products of maleic acid or fumaric acid and polyethylene glycol and/or monoalkyl ether of polyethylene glycol.
2. Discussion of the Background
The use of a polyoxyethylene-containing maleic acid ester as a stabilizer for an aqueous polymer dispersion is known. For example, German OS 27 54 140 discloses polymeric emulsifiers obtained by reacting polybutadiene-maleic anhydride copolymers with polyethylene glycols and/or monoalkyl ethers of polyethylene glycol. However, the production of such a comb polymer is a multi-step process which involves producing the polymeric fundamental structure and then reacting with polyethylene glycol and/or its monoalkyl ether derivative. It is very difficult to control the product, particularly in this last reaction.
Accordingly, it is desirable to directly synthesize a surface-active comb polymer. German OS 15 70 805 and European OS 0,248,612 describe hydrophobic monomers copolymerized with ethylenically unsaturated derivatives of polyethylene glycol. However, this process suffers from the disadvantage that the protective colloid must be produced in a separate reaction, which is costly. This disadvantage can be avoided by producing the protective colloid in situ at the time of the emulsion polymerization.
European OS 0,013,478 describes a method wherein ethylenically unsaturated hydrophilic oligomers and/or polymers, generally based on (meth)acrylic acid esters and commonly referred to as "macromonomers", are copolymerized with additional monomers in an aqueous phase. The known incompatibility of the polyoxyethylene chains with the predominantly hydrophobic polymer main chains, along with a statistical incorporation of the macromonomers, ensures their distribution over the surface of the latex particles, resulting in stabilization of the polymer dispersion.
According to European OS 0,013,478, the statistical incorporation of macromonomers based on (meth)acrylate esters entails difficulties when the polymerization is carried out in a purely aqueous emulsion without using an organic cosolvent. When a water-soluble initiator is employed, these water-soluble macromonomers homopolymerize in a purely aqueous phase, resulting in poor stabilization of the latex particles. As a result, the method according to European OS 0,013,478 employs added organic cosolvents.
In addition, such macromonomers based on (meth)acrylic acid are produced using an inhibitor in order to avoid polymerization during the condensation reaction. These inhibitors must then be removed, at additional cost, to prevent their interference with the subsequent emulsion polymerization.
The good water-solubility of the macromonomers is not disadvantageous if they do not homopolymerize. If such a condition exists, one can compensate for unfavorable concentration ratios of the comonomers in the aqueous phase. In this connection, the advantageous copolymerization behavior of maleic acid and its derivatives is known. German Patent No. 1,100,958 disclose polyoxyethylene-containing maleic acid monoesters or mixed maleic acid polyethylene glycol alkyl esters as comonomers in producing polymer dispersions. In order to achieve a good emulsification effect, long chain substituted omega-alkyl polyethylene glycols or omega-(alkylaryloxy) polyethylene glycols are used which are strongly surface active. However, such additives must be produced in a separate multistage chemical reaction process which is costly and time consuming.
German OS 33 90 093 disclose aqueous polymer dispersions produced without the usual suspending or emulsifying agents. The dispersions contain a polymer which is a copolymer of at least one amphipathic and at least one nonamphipathic monomer. The lipophilic component of the amphipathic monomer has a molecular weight of at least 100 and contains at least one ethylenic double bond. If one produces the amphipathic monomer starting with maleic acid or fumaric acid, the necessary lipophilic character can be conferred by converting it into derivatives having hydrocarbon chains, e.g., by esterification with fatty alcohols to the monoester stage. In addition a polymeric water-insoluble component such as polyoxypropylene or polyoxybutylene can be grafted thereon. Example 4 of the above-cited document apparently combines both techniques.
The amphipathic monomers used according to German OS 33 90 093 have an emulsifying effect. Due to the naturally incomplete incorporation in the polymerization, the latex always contains a residual amount of such molecules, which makes it difficult to achieve a high surface tension necessary for certain applications. A further disadvantage results from the fact that such latexes tend to be substantial foam-formers.